Charge Transfer-Induced Complexation of Urea Confined Within 1,4-Dimethoxypillar[5]arene

The synthesis, molecular structure, and spectral characteristics of inclusion complexes of 1,4-dimethoxypillar[5]arene (DMP5A) macrocyclic host with 1-(2-chloropyridin-4-yl)-3-phenylurea (ClPU) and 1,3-bis(4-chlorophenyl)urea (ClU) guests are characterized by ¹H/2D NMR, infrared spectroscopy, high-resolution mass spectrometry, and steady-state fluorescence experiments alongside ωB97x-based density functional theory (DFT). The molecular structure of the ClU⊂DMP5A complex predicted from the present DFT calculations reveals that the ClU guest is completely encapsulated within the cavity of DMP5A. The ClU⊂DMP5A complex is stabilized by C-H···π interactions between the methoxy protons of DMP5A and the aromatic rings of ClU, together with displaced π-π interactions, visualized as green sheet-like regions in noncovalent interaction-reduced density gradient analysis. On the other hand, the pyridine ring of ClPU interacts primarily with the methoxy substituents of DMP5A. The calculated interaction energy of ClU⊂DMP5A (-133.2 kJ mol^-1) is ≈2.9 times greater than that of ClPU⊂DMP5A, indicating significantly stronger binding. Time-resolved fluorescence measurements show a reduced average lifetime for ClU⊂DMP5A, consistent with ground-state complex formation. Cyclic voltammetry further demonstrates donor-acceptor Lewis-type interactions, with the fully encapsulated ClU⊂DMP5A complex exhibiting a two-electron transfer process, highlighting the influence of guest encapsulation on host-guest binding and charge transfer behavior.